A portable telephone system, such as the second-generation digital cordless telephone (CT2) system, has multiple transceivers (providing multiple RF channels) which are located at a call point station or public base station known as a telepoint. These transceivers allow persons using portable telephones or cordless telephone handsets to access the public telephone network when in range or within a service area after the cordless handset has established a synchronous link with the base system.
In the CT2 system, the cordless handset initiating a call, asynchronously transmits on one available channel of the handset's transceiver which corresponds to a transceiver radio frequency (RF) channel of the base. For a good understanding of the CT2 system communication protocol one is referred to a publication entitled, "Common air interface specification to be used for the interworking between cordless telephone apparatus in the frequency band 864.1 MHz to 868.1 MHz, including public access services", Version 1.1, dated Jun. 30, 1991, which is published by the European Telecommunications Standards Institute and is hereby incorporated by reference. The present standard for cordless telephones is publication entitled, "Common Air Interface Specification To Be Used For The Interworking Between Cordless Telephone Apparatus Including Public Access Services., MPT 1375: May 1989", Amendment No. 1, dated November 1989, published by the Department of Trade and Industry, London, England, and which is hereby incorporated by reference.
In the typical CT2 system three main communication protocols, called multiplex 3 (MUX 3), multiplex 2 (MUX 2) and multiplex 1.4 or 1.2 (MUX 1.4 or MUX 1.2) are utilized for communication between the cordless handset (or as referred to in the industry cordless portable part, "CPP") and the base station (or cordless fixed part, "CFP"). MUX 3 is utilized mainly for communication link initiation (link establishment and reestablishment) between the CPP and CFP. The MUX 2 protocol is used primarily for communication link establishment and for link initiation from the base (CFP). While the MUX 1 protocol is used primarily for voice/data communications, signalling information, and control messages from the CPP (portable) and CFP (base).
In the CT2 system, the cordless handset initiating a call, asynchronously transmits on one available channel of the handset's transceiver which corresponds to a transceiver radio frequency (RF) channel of the base.
According to one protocol out of other applicable protocols, called multiplex 3 (MUX3) of the CT2 specification, the channel used by the cordless handset is first divided in the time domain into 7 frames, F1-F7, as illustrated in FIG. 1a. Accordingly, the cordless handset transmits continuously for five frames or transmission bursts of ten milliseconds and the receiver receives for four milliseconds in a receiving time-slot window, when the transmitter is turned off for two frames. Within the ten milliseconds of transmit time comprising 5 frames, the information is repeated four times (in each submux) within a two millisecond frame, as seen in FIG. 2.
A typical layout of the MUX3 format in a sequential order of the frames is shown in FIG. 2. The labels F1 through F7, respectively, indicate frame numbers. The period of a single frame is two milliseconds. Referring to both FIGS. 1b and 2, each frame, F1-F7, is divided into four smaller subframes s1-s4 containing differently sized data (D) or frame synchronization words (CHMP), each preceded by a different number of preamble bits (P). The entire synchronization and data information of the repeating data signal 16 is then repeated four times in each of the four subframes s1-s4.
All the subframes, where the complete information is periodically provided once, are grouped together to form a submultiplex or submux 40. Each frame thus consists of four submuxes 40, corresponding to the subframes s1-s4. Within each submux 40, a D channel synchronization character (SYNCD), 3 address code words (ACW), and a frame synchronization character of twenty-four bits (CHMP) exist. Each of these information words are preceded by some number of preamble bits. Each subframe or submux consists of thirty-six bits, in a row. Finally, the first two bits of the 8 bit preamble word of the next 10 bit SYNCD data word ends the first row of the first submux or subframe s1. The rest of the D channel data are arranged accordingly, as seen in FIG. 2.
On the other end as seen in FIG. 1b, the base station asynchronously receives (14) and transmits (12) alternately for one millisecond in a time domain duplex (TDD) burst mode, after a synchronous link has been established with another CT-2 protocol MUX1 or MUX2. While scanning through the different radio frequencies of its transceivers, the base station looks for the presence of a synchronization signal (i.e. the frame synchronization character CHMP) to determine if that channel or frequency is used by a handset in a call attempt.
Acknowledged and unacknowledged information transfer over the link occurs through codewords sent in the D channel at regular intervals. The general codeword structure, depicted in FIG. 11, contains 64 total data bits. Address codewords (ACW) are preceded with a 16 bit synchronization word (SYNCD) to enable the receiver to establish codeword framing.
In FIG. 3A, a simplified MUX 3 link initiation packet is shown. The two other important communication protocols utilized in a CT-2 communication system are MUX 2, shown in FIG. 3B, and MUX 1 (implemented either via MUX 1.2, 2 bit signalling, or MUX 1.4, 4 bit signalling), shown in FIG. 3C. MUX 2 is used after link initiation under MUX 3 has been achieved. MUX 2 is used to carry the D, or signalling channel information, and SYN, or synchronization channel information, for communication link establishment and re-establishment. B channel information which carries the 32 kbit/second speech or data is nonexistent in MUX2. MUX 2 carries the D channel at a data rate of 16 kbit/second and the SYN channel at an overall rate of 17 kbit/second. The SYN channel consists of ten bits of preamble (one-zero transitions) followed by a channel marker (CHMF) or synchronization marker (SYNC). Data bits in the D channel are aligned in MUX 2 so that the D channel synchronization word, SYNCD, occurs as the first 16 bits in the D channel after the SYN channel.
In FIG. 3C, the MUX 1 signalling packet is shown. MUX 1 is envoked from MUX 2 by a control message. MUX 1 is used bi-directionally over an established link to carry D channel (Data) and B channel (speech/data). The SYN (synchronization) channel is nonexistent in this multiplex and therefore should burst synchronization be lost, it cannot be recovered without reinitializing the communication link. MUX 1 supports both 68 bit (referred to as MUX 1.4) or 66 bit (referred to as MUX 1.2) burst structures. It is up to each system manufacturer to decide which of the two to utilize for MUX 1 signalling. MUX 1 data rates are 2.0 kbit/second for MUX 1.4 and 1.0 kbit/second for MUX 1.2. Data bytes in the D (data) channel are aligned in this multiplex so that bytes always start on a frame boundary.
As can be seen from the above discussion, if after a communication link between a cordless telephone and a CT-2 base station is lost after the link is in MUX 1 (voice communication has begun) or in MUX 2, the units must both return to MUX 3 to resynchronize with each other. The present link re-establishment protocol in CT-2 systems has been found not to provide the level of performance that is required for such systems. Typical problems experienced with present CT-2 systems include "selective multipath" which causes loss of link synchronization in MUX 1 and thereby requires link re-establishment in order to continue voice communications. Another major problem is caused by another communication unit operating in an asynchronous mode occupying an RF channel closely spaced to the RF channel being utilized by the cordless telephone. Finally, given the low power (10 milliwatt) maximum power output of the CT-2 portable unit, communication links can be lost by a portable user moving out of range of the base station. All of the above mentioned problems cause the communication link to fail by causing loss of synchronization in MUX 1 between the base station and portable, thereby requiring link re-establishment.
A need exists in CT-2 systems for a communication link re-establishment protocol which can provide more reliable and efficient link re-establishment after loss of a communication link between a CT-2 portable unit and a CT-2 base station.